This invention relates generally to high resolution color cathode ray tubes (CRT's) and specifically to color cathode ray tubes of the index type.
Index color CRTs are well known for their excellent resolution capabilities which are generally much superior to those of the more common color CRTs which have a shadow mask electrode interposed between the electron sources in the tube neck and the groups of colored light-emitting phosphors on the tube faceplate. The shadow mask functions in cooperation with the mechanically or electrically offset electron sources to shield each different "color" phosphor from electrons emanating from all but its respective source, or as is more commonly referred to, electron gun. Shadow mask CRTs are used extensively in commercial television receivers. For many video and graphics display systems, for an ever increasing number of computer terminals, and for many instrumentation uses, a much greater degree of resolution is desired than is obtainable from shadow mask CRTs. The single beam index type color tube is ideally suited to such demanding uses because of its resolution capability and because, at any time, the portion of the phosphor screen being bombarded by the electron beam is known.
The conventional single beam index color CRT includes a faceplate having a pattern of vertically oriented stripes of different colored light-emitting phosphors on its inner surface with each phosphor stripe being separated from its neighboring phosphor stripe by a guard band of inert material, that is material that does not emit light energy under electron bombardment. As is well known, the guard band enables selective energization of the different color phosphors and allows the size of the electron beam to be larger than the width of the phosphor stripes for maximizing the CRT brightness capability. The stripes are arranged in a regular series of colored light-emitting phosphors--generally R, G, and B-corresponding to a stripe of red light-emitting phosphor, a stripe of green light-emitting phosphor and a stripe of blue light-emitting phosphor. The phosphor stripes and the stripes of inert material are collectively referred to as a screen. The side of the screen closest to the electron source is covered with a vacuum deposited layer of aluminum which not only forms a conductive surface for the screen, but also reflects backwardly directed light emitted by the phosphors toward the faceplate to further enhance brightness of the CRT image.
Also in accordance with conventional index tube construction, special signal areas are disposed at regular intervals on the screen for generating index pulses responsive to impingement by the scanning electron beam. The special areas may comprise narrow index strips of energy-emissive material deposited over the aluminum layer and centered over every third stripe of inert material. The energy-emissive material, responsive to electron bombardment, produces detectable energy. While many types of energy-emissive material may be used, ordinary phosphor is suitable. An excellent phosphor for this use is the common type known as P47 whose light output is bluish and near ultraviolet in the energy spectrum and which has approximately equal rise and fall times. The preferred embodiment of this invention uses such a phosphor, and its light is directed toward the neck of the tube where it is sensed by a photo multiplier tube (PMT) positioned outside of and adjacent to the rear of the CRT envelope. The aluminum layer prevents light emitted by the index strips from reaching the front of the tube.
As the electron beam is deflected across the phosphor screen, the pulses of energy emitted by the index strips are detected by the PMT and processed to form an index signal. As reference to the copending application will show, the index signal, a clock signal, is used to control the delivery of beam modulating video information to assure that the beam is "aimed" at a phosphor stripe of correct color. The clock signal also is used to accurately determine the location of the electron beam on the phosphor screen for proper positioning of displayed video data.
One of the problems addressed in the copending application is that the electron beam, under certain drive conditions, significantly impinges an index strip as well as a color phosphor stripe. This is particularly troublesome when heavily bombarding the color stripes immediately adjacent to an index strip. Under some conditions the index strip pulse may be totally masked by electrons in a beam directed at the adjacent color stripe. The prior art has resorted, in many instances, to narrow-band systems to overcome this difficulty. Such systems introduce other difficulties, especially in the area of scan control which must be made more precise. On the other hand, the present system and the above referenced system in the copending application are wide-band systems which are very tolerant of scan since the index pulses are monitored on an almost instantaneous basis from one index strip to the next. Unlike narrow-band systems which tend to average index pulses to generate an index signal, the wide-band system disclosed in the copending application acts on each index strip and inhibits the generation of an index signal whenever video information is supplied to the CRT.
Another problem in index tubes is that contrast of the displayed image is impaired because of the need for beam current to excite the index strips to generate the index signal, even when there is no video information. The system of the co-pending application additionally discloses means for enhancing the contrast of the displayed image by cutting the beam current off between index strips in the absence of video information and for turning the beam on at a low level when an index strip is anticipated. These techniques are feasible because of the fast response time of the wide-band system which enables reaction to each index strip pulse or the absence thereof.
However, even in a wise-band system the beam energy may need to be severely restricted, at the expense of CRT performance, to enable accurate index signal generation. Thus the resultant limitation on brightness to assure accurate index signal generation may make the tube unacceptable for many applications. Suffice it to say that there is a need to be able to generate accurate index signals even in the face of heavy electron bombardment of adjacent color phosphor stripes.